The invention relates to a process for producing polyurethane moldings, wherein the reaction components are mixed under high pressure in a mixing head, the pressures and the positions of the mixing head slides or of the injector needles and the positions of the hydraulic valves are recorded, registered and evaluated by a monitoring device.
The production of polyurethane moldings can be undertaken in accordance with the so-called RIM (Reaction Injection Moulding) process, i.e., with molding tools in which the upper and lower parts of the mold have been moved together and consequently form a closed cavity. In this case, the mixing head with its outflow system is connected to the cavity via a runner. But the production of polyurethane moldings may also be undertaken with molding tools in which the upper part of the mold is first opened and the polyurethane reaction mixture is charged into the open cavity via the outflow system of the mixing head, and the upper part of the mold is closed after the end of the shot, that is, the mold is closed.
In the RIM process according to the state of the art (shown in FIGS. 1 and 2), at least two liquid components that react with one another are supplied by a metering machine to a mixing head in a predetermined mass-flow ratio via pipelines or hose lines. The mixing head has the task of mixing these components, as a rule isocyanate and polyol, homogeneously with one another and of emitting the polyurethane reaction mixture arising in the process via a runner, which is generally an integral part of the mold, into a mold (molding tool). The liquid reaction mixture that has been transferred into the mold cures after some time. After this, the mold, which as a rule is of multi-part construction, can be opened in order to remove the finished molding. Generally, the mold is integrated within a mold carrier (not illustrated) which opens and closes the mold and, in the closed state, keeps it closed by acting against the internal pressure arising in the mold. In the case of the RIM process, the mixing head is, as a rule, permanently attached to the molding tool. But it is also possible to dock the mixing head onto the tool prior to each shot with a special mixing head portal, or manually. The mixing heads that are employed in this case are so-called high-pressure mixing heads which operate, for example, in accordance with the counterflow injection process.
The production of the molding can be subdivided into phases that take place in succession. At the start of the production process, the hydraulically driven slide which is located in the interior of the mixing head is located in a position that permits the recirculation of the components via the mixing head. This means that the mass flow of the components, which is metered by the metering machine, flows to the mixing head via the inlet. In the mixing head the flow of the components is channelled by grooves and channels into the return flow. From there the mass flows of the components flow back to the metering machine. The channels in the mixing head which are flowed through by the components may also be formed by a different geometrical arrangement of mixing head casing and slide, or even by mixing head casing and several slides. The construction of the mixing head that is illustrated in the figures is only one possible variant. Starting from an empty closed mold (FIG. 1), one or more hydraulically driven slides are moved in the high-pressure mixing head. These slides have the task of channelling the fluid flows of the components to be mixed in such a way that they are combined and a homogeneously mixed reaction mixture arises which emerges from the mixing head and is emitted into the mold. In this connection, so much reaction mixture is emitted that the mold is filled (FIG. 2).
In order to emit the exact quantity required into the molding tool, the metering of the reaction components is undertaken with the aid of a metering machine. This machine meters each component at a rigidly defined mass flow-rate. All the components are metered for the same established period. This period is monitored via an electrical control system. The start and the end of the metering process are controlled by the movement of one or more hydraulically moved slides in the mixing head. After the mixing and filling process, the hydraulic slide or slides move(s) in such a way that residues of the polyurethane reaction mixture are totally ejected from the mixing head and the mass flows of the components recirculate again via the mixing head. Subsequently, the reaction mixture which has been introduced into the mold cures, and a solid polyurethane molding is formed which can be removed from the mold after the mold has been opened.
In the course of RIM production with add-on mixing heads, serious damage to the plant may occur for many reasons which lie outside the region of metering (metering machine, mixing head). For example, it is possible that residues of a finished molding in the sprue region of the mould are forgotten in the course of demolding. As a result of this, the outlet of the mixing head then becomes sealed, and during the metering process an inadmissible pressure arises in the component lines which connect the metering machine and the mixing head. This then results in the malfunction and automatic shutdown of the entire plant because the maximum admissible pressure has been exceeded. The shutdown is effected by the electronic control system of the plant.
Such a shutdown by reason of exceeding maximum admissible pressure may also occur as a result of problems in the metering area (metering machine, mixing head). For instance, it is possible for a nozzle in the mixing head to become clogged by particles of dirt. This brings about a higher flow resistance and leads to the exceeding of the maximum admissible pressure. Problems that are caused in the region of metering (metering machine, mixing head) can, as a rule, be recognized by the operator of the machine and also result in no damage to the plant when the plant is switched on again. In contrast, problems caused outside the system constituted by metering machine and mixing head may result in damage when the plant is switched on again.
If, for example, the runner is clogged, the pressure in one component line rises more quickly, for example by reason of a higher discharge capacity, than in the other. Therefore a partial flow of component is forced from the component line having the higher pressure into the line having the lower pressure via the mixing chamber 9 (FIG. 2) which is filled with reaction mixture. This is possible because the component lines expand in the event of a rise in pressure and consequently enlarge their volume. If, for example, the pressure in the isocyanate line rises more quickly than the pressure in the polyol line, isocyanate is able to enter the polyol line. This then results in the formation of a reaction mixture in the polyol line, and solid polyurethane is formed. If the plant is put into operation again after this occurs, the reaction mixture is entrained through the entire plant by the recirculation of the components. The result of this is that all of the hose lines and pipelines become clogged with solid, fully reacted polyurethane. If the switching-on of the plant again after such a fault were to be prevented, only the component lines that are located in the vicinity of the mixing head would be affected.
RIM plants known hitherto have been equipped with an automatic pressure monitoring system for the purpose of protecting the plant. This system shuts down the plant automatically if a maximum admissible pressure is exceeded in one of the component lines. However, this pressure shutdown only takes account of the result of the exceeding of the pressure, irrespective of when and in what connection it has occurred. This means that other parameters of the plant are not taken into account and cannot be considered in connection with the pressure shutdown, so that for the operator of the plant it is not evident where the cause of an automatic shutdown by reason of an exceeding of the admissible pressure is to be sought.